The Guido Fawkes political website is screened out by TalkTalk, if users have requested social media blocking, because it is classified as a blog. Photograph: www.order-order.com

Nearly one in five of the most visited sites on the internet are being blocked by the adult content filters installed on Britain's broadband and mobile networks.

A Porsche car dealership, two feminist websites, a blog on the Syrian War and the Guido Fawkes political site are among the domains that have fallen foul of the recently installed filters.

The Open Rights Group, which campaigns for digital rights, surveyed the 100,000 most popular sites as ranked by digital marketing research firm Alexa, and found that 19,000 of them were blocked by at least one fixed line or mobile internet service provider.

The UK's four mobile networks have used filters for a number of years, with some imposing them on all pay-as-you-go subscribers.

Following a push by David Cameron, broadband companies including TalkTalk, BT, Virgin Media and Sky have caught up, introducing adult content blocks that offer parents the choice of screening out potentially harmful content. Every household will be asked whether they wish to install a broadband filter before the end of 2014. The filters screen out pornography, suicide and self-harm related content, weapons and violence, gambling, drugs, alcohol and tobacco, but users can also opt to block dating, music and film piracy, games and social networking.

"Filters can stop customers accessing your business, block political commentary or harm your education," said Jim Killock, executive director at Open Rights. "The government has told everybody that they have to take child safety extremely seriously and that filters are in some way an answer to that. People are being pushed into filtering lots of content that they simply don't need to and is not dangerous to children."

Marielle Volz, an American who had just moved to the UK with her husband and young son, had no internet access at home and wanted to read an article about recovering from childbirth on her mobile phone.

But the feature, "Stop acting like 'bouncing back' from labor is even possible", was published by Jezebel.com, a website owned by the respected digital publisher Gawker Media, and which is currently blocked by her mobile network, Three.

Volz said: "I was so excited to move to a country with sane maternity leave policies, only to find I couldn't even read an article about it!" Three imposes an adult content filter as default for all pay as you go customers.

A spokesman for the network said the Jezebel block was under review and that customers could report any wrongly filtered material via its website.

Philip Raby, who runs a Porsche brokerage based in Chichester, found his website had fallen foul of O2's filter. He sent emails and made calls, but was unsuccessful in having the ban lifted until he began tweeting about the problem. "We must have lost some business as a result," said Raby. "It doesn't look great telling people the site is not suitable for under 18s!"

A spokesman for O2 said: "The vast majority of sites are categorised correctly but with the millions of sites now connected to the web and the wide variety of content, mistakes can happen."

Syrian War commentator Aboud Dandachi's blog has been screened out by EE, O2, Sky and Vodafone, according to Open Rights Group's research, although a spokesman for Vodafone said it was now visible on his filtered phone.

Sherights.com, which focuses on sexual health, violence against women and lesbian and gay rights, relies like many sites on traffic to its pages to earn the advertising revenue that funds its content. It was blocked by TalkTalk in April, and the bar has since been lifted.

Sherights editor Maureen Shaw said: "The effect of filtering boils down to advertising revenue, which is based on site visitors. If people who would normally be interested in accessing our content are not able to view the site, it directly impacts our bottom line."

The Guido Fawkes website is being blocked in households which have selected to screen out all social media, according to TalkTalk. The category covers Facebook and Twitter, but also blogs and chat forums, and www.order-order.com is classified as a blog.

TalkTalk, whose filter is called HomeSafe, said customers could email Homesafe.classification@talktalkplc.com to report wrongly blocked sites. A spokeswoman said: "We welcome feedback to ensure we are continually improving the product."